Allow histogram to show one histogram per cube slice

[astrofrog]

When working on #593 I noticed that in the case of a data cube, the histogram shows all slices in a single histogram. For e.g. PPV cubes this might make sense, but for e.g. an RGB cube it does not. Therefore it would be nice to have an option to select whether to show all pixels, and if not, to have a slider to 'slice' through the histogram.